The present invention relates to a print head for a dot matrix printer in which each print needle is separated from an armature, and more particularly to a return coil spring for returning the print needle in the print head.
U.S. Pat. No. 4,647,236 discloses such a print head in which a needle return spring is provided between an impact head of the print needle and a guide member formed in a guide nose.
Generally, the impact head is manufactured by a cutting process with a lathe. The manufactured head is fixed to a needle body by brazing or spot welding or other processes. However, such a manufacturing process comprises a plurality of steps, which causes an increase of manufacturing cost.
In order to produce a print needle at low cost, a heading process (cold forging) for integrally forming the head with the needle body is proposed. However, the diameter of the impact head manufactured by the heading process inherently becomes small. In the prior art, the return spring is a coil spring having a constant diameter. Accordingly, the diameter of the coil spring becomes small with the reduction of the diameter of the impact head. However, it is difficult to assemble such a small diameter spring into a print needle. In operation, the print needle slides on the spring at the inside periphery thereof, which causes an unstable operation of the needle because of sliding resistance. Since the diameter of the spring is small, an excessive force of the spring at the compression thereof is exerted in a bottom wall of the guide member, so that the bottom is worn by the end of the spring to form a recess. As a result, returning force of the spring decreases, causing malfunctions of printing operation, such as scratching of an inked ribbon with the print needle.